This invention relates generally to a method and system for measuring table sag, and more particularly, this invention relates to an improved method and system for measuring sag of a loaded table used in imaging systems.
In a computed tomography (CT) system, an x-ray source projects a fan-shaped beam that is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon a multi-row multi-column detector array. The detector array comprises a plurality of detector elements. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector element of the detector array produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all of the detector elements are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of both the x-ray source and the detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of the corresponding pixel on a cathode ray tube display.
A detector array in a CT imaging system comprises a plurality of detector modules. Each detector module has a scintillator array optically coupled to a semiconductor photodiode array. The scintillator array emits light in response to receiving x-rays. The photodiode array detects light output by the scintillator array and generates electrical signals responsive thereto.
When acquiring CT images, it is important to have precise patient and image registration. For radiation treatment (RT) planning purposes, it is desired to scan the patient on the CT scanner in the exact position that will be used for radiation treatment. Third party laser lights are often installed and calibrated in order to assist with this patient positioning. The 3rd party laser lights are usually installed a substantial distance (˜0.6 m) from the scanning plane and a noticeable amount of vertical table sag can occur between the table position used to align the patient with the third party lights and the table position used while acquiring the CT scan. This table sag, if unaccounted for, could result in errors in the radiation treatment. Table sag distances as large as 6 mm have been measured at clinical sites.
U.S. Pat. No. 6,561,695 addresses several patient and image registration inaccuracies, including table sag. However, the method requires special equipment to be installed in tables. Thus, previous methods have used additional hardware that gives geometrical information including table sag and special tables must be purchased and installed for conducting such methods.